The present invention relates to the interconnection of transmission lines, and particularly to the overlapped interconnection of coplanar transmission lines (coplanar waveguides) so as to minimize variations in the impedance of the interconnection due to possible transverse and/of longitudinal misalignment of the connected elements.
In high-frequency test fixtures, probes, and the like, and in the packaging or mounting of high-frequency chips, devices or circuits, it is often necessary to make temporary or permanent connections between coplanar transmission lines. Often these interconnections must be smaller or less expensive than is obtainable using a conventional connector. In such cases, it has sometimes been convenient to interconnect a pair of coplanar transmission lines by abutting them end-to-end and bridging their juncture by means of a parallel array of thin, closely-spaced conductive strips mounted on a dielectric substrate and overlapping the signal and ground lines of both transmission lines. However, this structure requires a separate connecting piece containing the thin conductive strips. A alternative type of connection is a longitudinally overlapping interface between upward-facing ground and signal lines on one transmission line and downwardfacing ground and signal lines on the other. This type of interface is more attractive because it requires no extra connecting piece.
However, a problem with both the end-to-end abutment and longitudinally overlapping types of interfaces is that any variations in transverse or longitudinal alignment of the transmission lines result in corresponding variations in the characteristic impedance of the interface, defeating the constant characteristic impedance normally desired in a transmission line and causing undesirable reflections and distortions of high frequency signals. Although this problem has been addressed to some extent in the connection of an electrical component to a stripline transmission line having signal and ground conductors on opposite sides of a dielectric substrate as shown, for example, in U.S. Pat. No. 3,218,584, the problems and solutions relevant to stripline transmission lines are not applicable to the interconnection of coplanar transmission lines. Accordingly, what is needed is an interface structure for connecting a pair of longitudinally overlapping coplanar transmission lines which permits transverse and/or longitudinal misalignments thereof without causing significant variations in the characteristic impedance of the interface.